One option is to use the Theme Switcher (in Vandelay Industries module) - you can switch to different themes based on Url and other rules, or write your own rules. A theme can have a BasedOn property, so your base theme has a standard set of templates, and
you can override just the templates you want to in derived themes.

There are other options like using Placement.info to specify alternate template filenames based on content type or Url. Personally I recommend overriding templates based on Content Type if you can, this gives you the most ease and flexibility, and won't
mess things up if you, for instance, change the Url of a page but want it to keep the same look.